The present invention :.elates to a radar antenna arrangement and a method, using the antenna arrangement, for determining the effective radar backscatter cross section of a radar target as a function of the geometrical position of the target relative to the radar antenna.
In order to determine the behavior of the radar reflection cross section .sigma. of a target as a function of the aspect angle, it is known to measure the radar energy scattered back from the target as a function of the aspect angle (for a constant distance and a constant elevation angle). However, in this method it is assumed that the target is fully illuminated and is disposed in the far field of the receiving antenna.
The term "radar backscatter cross section" is defined particularly for such a case; the target acts as a single reflector (point source). If the expanse of the target is significantly greater than the wavelength, the radar backscatter cross section is greatly dependent on the aspect angle. This angle dependence is independent of the distance.
If the target is not fully illuminated by the radar beam, for example, because the antenna is sharply focused, the target must be scanned by the radar beam, with only part of the target being illuminated during each scanning process. The radar backscatter cross section measured in this case does, of course, not describe the radar backscatter behavior of the entire radar target, but only that part of the target that is illuminated by half to all of the 3dB antenna beam area. This is then called the effective radar cross section .sigma..sub.eff. This term is also employed if the receiving antenna is disposed in the near field of the illuminated target or target section. The near field is that distance R for which the following applies: ##EQU1## where a=the diameter of the target and .lambda.=the wavelength.
Knowledge of only one value oeff, measured for a given position of the target relative to the radar device, does not permit a conclusion as to oeff values for other positions of the target relative to the radar device, particularly if the distance or the resolution cell varies, e.g., because other parts of the target are illuminated. Such a problem exists, for example, if the behavior of a rotating search head of an ammunition body that is descending on a parachute and scans a target area with the aid of a millimeter-wave radar device is to be determined. In that case, the phase point of the antenna in space changes continuously, on the one hand, and, due to the intensive bundling of the radar beam, on the other hand, the target is illuminated only partially. Finally, the wavelength is generally much smaller than the target dimensions, e.g., a tank, and the receiving antenna is located in the near field of the target.